The long-term goal of this research program is to elucidate the mechanism of action of thyroid hormones at the cellular level through a study of the relationships between hormone-receptor interactions, hormone actions and hormone metabolism in certain amphibian species: Rana catesbeiana, Xenopus laevis, Necturus maculosus and Ambystoma mexicanum. For a variety of reasons these animals offer a unique opportunity to investigate these relationships. Past studies have been conducted primarily in pre- and prometamorphic Rana catesbeiana tadpoles and the presence of saturable, high affinity binding sites for thyroxine (T4) and 3,5,3'-triiodothyronine (T3) has been demonstrated in hepatic and tail nuclei at these stages. The studies proposed in this application are directed towards establishing the physiological relevance of these nuclear binding components, and determining the importance of T4, relative to T3, in the hormonal control of metamorphosis. The studies include: 1) direct measurement by RIA of the absolute amounts of T4 and T3 bound to saturable nuclear sites in tadpoles liver during metamorphic climax; 2) assessment of the extent to which monodeiodination of T4 occurs in this species during climax; 3) determination of the binding characteristics of saturable nuclear sites in tadpole liver during climax; 4) determination of the stage in the life cycle of Rana catesbeiana and Ambystoma mexicanum when saturable sites for T4 and T3 are first evident, and assessment of binding characteristics in adult, probably unresponsive, forms including the Necturus; 5) investigation of the relationship between occupancy of the nuclear sites by thyromimetic compounds and their biological effectiveness. This latter study will be carried out in tadpoles both in vivo, when latency and magnitude of biological response will be considered in relation to the duration of occupancy of nuclear sites, and in in vitro systems (isolated liver cells; cultured tail tips) in which the effectiveness of a hormone or analogue will be correlated with the percentage of the sites that are saturated with the compound.